Perceptual rules for watermarking images: a psychophysical study of the visual basis for digital pattern encryption

We have measured the contrast detection thresholds for small bandpass targets embedded in digitized monochrome photographs of natural scenes. The targets are used to probe the properties of watermarking patterns, which might be embedded in a photography to state copyright or authenticity, while remaining invisible to a human observer. Thresholds were measured for targets embedded in different parts of the photographs in order to determine where in a photographs it would be most suitable to hide a watermarking pattern. Thresholds were also compared when the photographs were bandpass filtered or notch filtered in order to determine how the localized spectral energy in the photograph affected the visibility of a potential watermarking pattern. We also studied the visibility of targets embedded in synthetic pictures, whose spectral amplitude was similar to that of natural scenes. The test targets were most easily visible when embedded in parts of photographs where the luminance was relatively uniform, and they were especially easy to see where the average luminance was low. This was explicable on a simple model of contrast encoding in the human visual system. The targets were much harder to see when embedded in contrast-rich parts of the digitized photographs. Indeed, the thresholds were evaluated more than the simple human model predicted: the spatially- localized contrast energy in the photograph masked the test target effectively. The experiments with notch-filtered photographs produced surprising results that were not predicted at all by the human model. Even when the spectral energy was removed from the photograph in the band occupied by the test target, there was still substantial masking. This implies considerable masking between visual primitives encoding different spectral bands. It also implies that watermarking technology might be facilitated, since any contrast energy may hide a watermarking target regardless of their respective spectral content.